1. Field of the Invention
This invention relates to methods and apparatus for continuously electroplating wire, and particularly to an improved method and apparatus for electrotinning copper wire at high speeds and with high current densities.
2. Description of the Prior Art
One known method of continuously electrotinning a copper wire employs stannous sulphate for the electrolyte. This method basically comprises passing a longitudinally moving copper wire a plurality of times through an electrolyte tank, each time passing the wire around an electrically-conducting driven grooved drum, which is connected to the negative pole of a d.c. current source, and a separate one of a plurality of insulated pulleys, the anodic current being supplied via anodes of tin immersed in the electrolyte. Typically the apparatus is such that the wire is horizontal when being planted, and it passes alternately through two plating tanks which are arranged one above the other between the pulleys and the drum. Using such an arrangement with stannous sulphate, electrolyte current densities of the order of 100 amps per square ft. (1076 amps per square meter) are employed, and the wire moves at a rate of 500 ft. (152 meters) per minute, the plating thickness being built up gradually during passage through the electrolyte.
Electrolytes capable of higher current densities of the order of, for example, 400 amps per square ft. (4,304 amps per square meter) are now available, for example stannous fluoroborate (also known as stannous fluoborate) and other fluoroborate-based plating solutions, with the result that with the same basic method and apparatus, plating rates four times greater than obtainable with stannous sulphate, for example, can be achieved.
The use of stannous fluoroborate or other fluoroborate based printing solutions, however, result in a number of practical problems. The highly corrosive nature of the electrolyte means that plastics materials or special steels need to be used for the tanks, pipework and pumps. In the above-mentioned practical arrangement for tin plating a copper wire using stannous sulphate, the tin anodes are positioned in the electrolyte tanks below the moving wire, but this produces problems, when using stannous fluoroborate, for example, in the method of connecting the anodes to their associated bus bars to that the latter are not corroded. In order to cope with the high current density (400 amps per square ft. or 4,304 amps per square meter) the wire must be arranged to pass in sufficient loops through the electrolyte tanks so as to pass the current without causing overheating.
In addition, the very much higher running speeds which can be achieved with fluoroborate-based solutions, for example up to four or more times the previous speeds, that is speeds of 2000 ft. (610 meters) per minute can cause hydrodynamic drag effects on the electrolyte in the tanks. With a large number of portions of wire passing through one tank in the same direction and at high speed, quite a considerable wave of electrolyte can be caused to move in this direction and spill over the end of the tank. This problem also occurs in connection with other processes, and the usual methods used to counteract it involve complex systems of weirs or other baffles.
It has already been proposed in our co-pending U.S. Pat. application Ser. No. 381,954, filed July 23, 1973, now U.S. Pat. No. 3,869,371, and assigned to the same assignee as the instant application, to overcome the above-mentioned hydrodynamic drag problem by use of apparatus which includes a single horizontal tank of electrolyte (for example stannous fluoroborate) and by passing the wire a plurality of times through the single tank, each time passing it in a loop around two rotatable drums between which the tank is positioned, spacing and deflector means being positioned between the bath and the drums so as to cause the portions of the wire loop sections passing through the bath to be in a substantially common horizontal plane with alternate wire loop sections moving in opposite directions. The effects of hydrodynamic drag in one direction are, therefore, substantially compensated for by the effects of drag in the other direction. In that case it was also proposed to dip the anode electrodes from above, in order to facilitate electrical connection thereto.
It has now been found that it is possible to use high current density electrolytes at high wire throughout speeds and at high plating currents without involving overheating and fluid drag problems, and without necessarily having to resort to the above-described design of plating equipment proposed by our co-pending application.